M. W. Cho

Origin of forward leakage current in GaN-based light-emitting devices

The authors fabricated GaN-based light-emitting diodes (LEDs) on two different GaN templates with the same LED structure. One on thin GaN template (∼2μm) with high dislocation density [low (109cm−2)] grown by metal-organic vapor-phase epitaxy (sample A) and the other on thick GaN template (∼20μm) with comparatively low dislocation density [high (108cm−2)] by hydride vapor-phase epitaxy (sample B). In order to understand the mechanism of leakage current in LEDs, the correlation between current-voltage characteristics and etch pit density of LEDs was studied.

The Fabrication of Vertical Light-Emitting Diodes Using Chemical Lift-Off Process

Vertical light-emitting diodes (LEDs) were successfully fabricated by a chemical lift-off process using a selectively etchable CrN buffer layer. The novel CrN metallic layer worked well as a buffer layer for growth of the GaN LED and was etched out clearly during selective chemical etching. The vertical LED by chemical lift-off showed very good current-voltage performance with low series resistance of 0.65 Omega and low operated voltage of 3.11 V at 350 mA. Also, this device could be operated at a much higher injection forward current (1118 mA at 3.70 V) by thermally conductive metal substrate which enabled the high current operation with excellent heat dissipation.

ZnO epitaxial layers grown on c-sapphire substrate with MgO buffer by plasma-assisted molecular beam epitaxy (P-MBE)

ZnO films on c-sapphire with and without an MgO buffer were grown by plasma-assisted molecular beam epitaxy. ZnO with an MgO buffer was two dimensionally grown, while ZnO without an MgO buffer was grown three dimensionally, which was confirmed by in situ RHEED (reflection high energy electron diffraction) and AFM (atomic force microscopy) observations. Morphology evolution and growth mechanism of an MgO buffer were studied by in situ RHEED observations. Mosaicity (tilt and twist angle), type and density of dislocation were studied by both TEM (transmission electron microscopy) and HRXRD (high resolution x-ray diffraction). Based on in situ RHEED observations, MgO buffer growth involves three important steps including two-dimensional (2D) growth (wetting layer), 2D–3D growth transition and 3D growth. The mechanism of MgO buffer growth can be attributed to three inter-related effects. They are lowering surface energy through a wetting process, creating nucleation sites through a 2D–3D growth transition, and reducing the defect density by introducing dislocation interactions. It was found that the surface morphology and structural properties of the ZnO layers were improved by employing a thin MgO buffer layer grown at around 500 °C followed by high temperature annealing at 800 °C. By introducing an MgO buffer, the formation of 30° rotational domains in ZnO layers was suppressed, and screw and edge dislocation density of ZnO layers was reduced from 6.1 × 10 8 cm−2 to 8.1 × 105 cm−2 and from 1.3 × 1010 cm−2 to 1.1 × 1010 cm−2, respectively.

Lattice strain in bulk GaN epilayers grown on CrN/sapphire template

Microphotoluminescence spectroscopy is used to investigate local strain in GaN films grown on c-sapphire with CrN buffer, where the CrN buffer is partly etched. Biaxial compressive strain dominates GaN films grown on CrN buffer/c-plane sapphire. The emission energies of D0X, FXA, and FXA-1LO emission lines shift gradually from a high value to a low one, as the excitation laser beam scans from the unetched side of the sample to the etched side, while the emission intensities show only a slight change. No cracking occurs in the GaN film during etching except a change in bending of the detached part of the GaN film from convex to concave as determined by surface profiler. Both the lattice parameter and the energy position of the bound exciton emission peaks from a completely detached GaN are almost the same as those reported for strain-free GaN. The line width of the (0002) ω scan of a GaN film is narrowed from 352 to 331 arc sec through detaching presumably due to decrease in bending. Those properties sugge...

Lattice relaxation mechanism of ZnO thin films grown on c-Al2O3 substrates by plasma-assisted molecular-beam epitaxy

We report on the lattice relaxation mechanism of ZnO films grown on c-Al2O3 substrates by plasma-assisted molecular-beam epitaxy. The lattice relaxation of ZnO films with various thicknesses up to 2000nm is investigated by using both in situ time-resolved reflection high energy electron diffraction observation during the initial growth and absolute lattice constant measurements (Bond method) for grown films. The residual strain in the films is explained in terms of lattice misfit relaxation (compression) at the growth temperature and thermal stress (tension) due to the difference of growth and measurement temperatures. In thick films (>1μm), the residual tensile strain begins to relax by bending and microcrack formation.

Structural and optical properties of high-quality ZnTe homoepitaxial layers

The structural and optical properties of high-quality homoepitaxial ZnTe films are investigated. A substrate surface treatment using diluted HF solution plays a key role in growing device-quality ZnTe layers. X-ray diffraction analysis of ZnTe epilayers based on the crystal-truncation-rod method suggests that a homoepitaxial ZnTe film grown on a HF-treated substrate can be regarded as an ideal truncated crystal without an interfacial layer, while a ZnTe layer grown on a substrate without HF treatment suggests the presence of an interfacial layer which may lead to degraded crystallinity of ZnTe overlayers. The crystal quality of the homoepitaxial ZnTe layers with HF treatments are characterized by an extremely narrow x-ray diffraction linewidth of 15.6 arcsec and dominant very sharp excitonic emission lines with dramatically reduced deep-level emission intensity in the photoluminescence (PL) spectrum. Three bound excitonic emission lines at neutral acceptors are observed in the PL from the high-quality ZnT...

Effects of interfacial layer structures on crystal structural properties of ZnO films

Single crystalline ZnO films were grown on Cr compound buffer layers on (0001) Al2O3 substrates by plasma assisted molecular beam epitaxy. In terms of lattice misfit reduction between ZnO and substrate, the CrN and Cr2O3/CrN buffers are investigated. The structural and optical qualities of ZnO films suggest the feasibility of Cr compound buffers for high-quality ZnO films growth on (0001) Al2O3 substrates. Moreover, the effects of interfacial structures on selective growth of different polar ZnO films are investigated. Zn-polar ZnO films are grown on the rocksalt CrN buffer and the formation of rhombohedral Cr2O3 results in the growth of O-polar films. The possible mechanism of polarity conversion is proposed. By employing the simple patterning and regrowth procedures, a periodical polarity converted structure in lateral is fabricated. The periodical change of the polarity is clearly confirmed by the polarity sensitive piezo response microscope images and the opposite hysteretic characteristic of the piez...

Self-separated freestanding GaN using a NH4Cl interlayer

Thick GaN films were grown on void buffer layer by hydride vapor phase epitaxy. The void-buffer layers were consisted of NH4Cl, GaN dots, and low-temperature (LT) GaN buffer layer. Instead of GaN, NH4Cl was easily synthesized in NH3 and HCl atmospheres by simply lowering the growth temperature to 500°C, and LT GaN buffer growth was followed during increasing substrate temperature to 600°C. The LT GaN buffer acted as a protecting layer against evaporation of the NH4Cl and a seeding layer for the high temperature (HT) GaN. The NH4Cl layer between a sapphire substrate with GaN dots and the LT GaN buffer were fully evaporated during the HT GaN growth at 1040°C. Many voids were formed at interface caused by evaporation of the NH4Cl layer, which strongly assisted self-separation of thick HT GaN during cooldown after the growth resulting in a 200μm thick freestanding (FS) GaN. The FS GaN showed smooth surface morphology and absence of any crack. The a-axis and c-axis lattice constants of FS GaN were 3.189 and 5....

Strain-free GaN thick films grown on single crystalline ZnO buffer layer with in situ lift-off technique

Strain-free freestanding GaN layers were prepared by in situ lift-off process using a ZnO buffer as a sacrificing layer. Thin Zn-polar ZnO layers were deposited on c-plane sapphire substrates, which was followed by the growth of Ga-polar GaN layers both by molecular beam epitaxy (MBE). The MBE-grown GaN layer acted as a protecting layer against decomposition of the ZnO layer and as a seeding layer for GaN growth. The ZnO layer was completely in situ etched off during growth of thick GaN layers at low temperature by hydride vapor phase epitaxy. Hence freestanding GaN layers were obtained for the consecutive growth of high-temperature GaN thick layers. The lattice constants of freestanding GaN agree with those of strain-free GaN bulk. Extensive microphotoluminescence study indicates that strain-free states extend throughout the high-temperature grown GaN layers.

Impact of V/III ratio on electrical properties of GaN thick films grown by hydride vapor-phase epitaxy

Impact of V/III ratio on electrical properties of GaN thick films are investigated, which are grown by hydride vapor-phase epitaxy. The authors note that the electron concentration of GaN films decreases with the increase of V/III ratio, while their electrical resistivity and electron mobility increase simultaneously. These indicate that enhancing V/III ratio suppresses electron-feeding sources in GaN films, which is not by generating electron-trapping centers but by reducing donor-type defects. On the other hand, it is shown that the linewidth of x-ray rocking curves in GaN films decreases and the near-band edge emission intensity of 10K photoluminescence spectra increases as V/III ratio increases. These mean that higher V/III ratio condition helps for reducing crystalline point defects in GaN films. In terms of theoretical fitting into the temperature-dependence curves of electron mobilities, it is found that the electron transport of GaN films grown in lower V/III ratio condition is more hampered by de...